I think you will find a wide variation on this forum regarding percentages. It would seem to depend on how active and robust your starter is. What temperatures you use for fermentation and length of time. I have also done 3%, 5% 10%, 15% etc. You just have to learn to tell when your dough is ready by visual clues (bubbles, etc), touch and smell. It is always an adventure and often fun to see how you can pretty much do the exact same method and come out with slightly different results which can be affected by oven temps varying, humidity, ambient temperatures, children screaming and crying - you get the picture!

Basically you need to experiment. I am using 5% of total dough weight and a 24 hour minimum rise @ 64F degrees. I then either bake it or chill it in the refrigerator for bread baking at a later date/time.